<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .big {
            position: relative;
            width: 400px;
            height: 400px;
            margin: 200px auto;
            
            background-color: blue;

        }
        .box {
            
            position: absolute;
            top: -75px;
            left: 150px;
            width: 0;
            height: 0;
            border: 50px solid transparent;
            border-bottom-color: blue;
        }
    </style>
</head>
<body>
    <div class="big">
        <div class="box"></div>
    </div>
</body>
</html>